Fish population densities in Otago's three largest lakes are relatively stable, new acoustic survey analysis suggests.

Otago Fish & Game has completed analysis for acoustic surveys of lakes Wānaka, Hāwea, and Wakatipu, which rank in the top 10 most-fished lakes in the country.

“Monitoring sports fish populations in big lakes is never easy,” Fish & Game officer Jayde Couper said. “These are big open spaces where fish can be widely dispersed.”

Jayde said the latest acoustic monitoring used a highly sensitive ‘echo sounder’, following methods established by previous surveys led by NIWA in 2007, 2008, 2009 and 2023.

This graphic illustrates fish densities at each lake over time. (The survey is not designed to compare lakes, which would require randomised surveying.) IMAGE: Otago Fish & Game

In the latest analysis, Lakes Hāwea and Wakatipu showed a small decrease from last year's fish density count, while Lake Wānaka increased on a low count last year.

A Fish & Game boat navigated precisely determined routes at various sections of the lakes to scan for fish at between three and 30 metres of depth.

“While it's not possible to survey the entirety of each lake, previous studies have shown the areas that hold the most fish and by repeating these areas over time, we can get a picture of how lake populations are trending,” Jayde said.

"Fish were found mostly near the lake bottom, similar to that observed last year. That makes sense because it’s generally where the food is.”

Fish densities were relatively stable across the five surveys covering 17 years.

According to the most recent national angling survey, the three big lakes make up more than 30 percent of angling in the Otago region.

A follow-up survey is planned for this summer to determine annual fluctuations and to build the long-term dataset to support management of the fishery.

This was the first survey conducted by Fish & Game staff and it followed extensive training by expert scientists at NIWA, Jayde said.